Chapter 18 – Excimer Laser Photorefractive Keratectomy (PRK)
WILLIAM J. LAHNERS
DAVID R. HARDTEN
• Photorefractive keratectomy (PRK) is a procedure in which the cornea is reshaped using an excimer laser. The procedure is designed to reduce the patient’s dependence on glasses or contact lenses. The excimer laser has revolutionized the field of refractive surgery.
The surgical treatment of myopia has made great strides over the last 15 years with the introduction and advancement of the excimer laser. The surgical treatment of astigmatism and hyperopia is also possible with the excimer laser. Its development has changed the face of refractive surgery more than any other technology in the history of ophthalmic surgery.
Ultraviolet radiation at the 193?nm wavelength can remove precise amounts of tissue from the anterior cornea. This precision overcomes in a large part one weakness of incisional refractive keratotomy: a lack of predictability. Use of the excimer laser at 193?nm for the treatment of low, moderate, and high myopia has now been well established.       The excimer laser can also be used to remove superficial anterior corneal scars    and to make hyperopic and astigmatic corrections.  Despite its promise in reshaping the cornea, the excimer laser is still unable to produce the perfect results that many expect with robotic laser surgery, and fortunately for the surgeon, there is still some surgical art required. 
More than 120 million Americans are currently dependent on glasses or contact lenses and may therefore be potential candidates for refractive surgical procedures, with even more potential candidates worldwide. Patients with anterior corneal scars whose vision is not improved by contact lenses or glasses usually are treated with penetrating keratoplasty, and excimer laser scar removal may reduce the need for this procedure in some patients. Advances in technology that improve the safety and predictability of refractive surgical procedures consequently may benefit a large number of patients.
Excimer laser photorefractive keratectomy (PRK) appears to be most reproducible for patients with -1.5 to -8.0D of myopia.      Higher degrees of myopia can be corrected, but this may be associated with higher degrees of regression or corneal haze.  Hyperopic corrections have taken more time to develop but are now routine for low to moderate levels. Low and moderate degrees of astigmatism can also be corrected with the excimer laser. 
HISTORY OF THE LASER
The acronym laser was given to this process by Schawlow and Townes and stands for light amplification by stimulated emission of radiation. The first clinically useful laser, using ruby as the medium, was developed in 1960 by Maiman  and was used in ophthalmology as early as 1961.  In 1961, the first continuous laser was developed by Ali Javan, with semiconductor lasers appearing in 1962 and liquid dye lasers in 1966.  Over the next several years, other lasers such as gas, solid crystal, and tunable dye lasers were developed.
Excimer lasers began in 1975 at Kansas State University, when Velasco and Setser noted that meta-stable rare gas atoms such as xenon (Xe) could react with halogens such as fluorine (F) to produce unstable compounds such as XeF under high pressures.   These compounds rapidly dissociated to the ground state of the individual molecules associated with the release of an energetic ultraviolet photon. These compounds could be made to undergo light amplification by stimulated emission when they were excited by an electron beam, with the argon-fluorine (ArF) molecule emitting light with a wavelength of 193?nm.
The word excimer (short for excited dimer) was first used to describe an energized molecule with two identical components. The argon molecule does not actually form an excited dimer, and the term rare gas halide or argon-fluorine more accurately describes the medium, but the term excimer has persisted.
Taboada et al.  noted that the ArF laser could produce opacification of the cornea and fluorescein staining with indentation at energy levels greater than 27.5?mJ/cm2 . Trokel et al. were able to demonstrate that the 193?nm laser could remove tissue very precisely in bovine cornea. About 0.25?µm of corneal tissue was removed with each pulse of the ArF laser, with minimal damage to surrounding tissues. The ablation thresholds, ablation rates, and healing patterns for different excimer wavelengths were described by Krueger and Trokel.    The ablation threshold for the cornea is the fluence at which tissue removal begins, which is approximately 50?mJ/cm2 for 193?nm. Below this fluence level, the cornea experiences only photochemical changes.
Munnerlyn[30A] described that when using PRK to change corneal curvature at a small optical zone, less tissue removal was needed to create the same change in curvature as when using a larger zone. The relationship can be simplified to:
For example, with a 6?mm optical zone, 4D of myopia is corrected with a 48?µm ablation depth. Early results showed clear corneas and good predictability of refractive effect in primate eyes. Blind human eyes were treated next, with promising clarity of the cornea and ability to change refraction.
Steepening of the cornea with the excimer laser to correct hyperopia was first introduced by L’Esperance et al. A larger ablation zone than that used in myopic PRK was needed to correct hyperopia. The optical zone is a relatively small portion of the ablated area, centered within a furrow created in the midperipheral region of the cornea. The treatment of hyperopia has lagged behind that of myopia because of the greater difficulty of steepening the cornea than flattening it, as required for myopic correction.
FUNDAMENTALS OF LASER MECHANICS
The electromagnetic spectrum is composed of a broad range of wavelengths, from long radio waves to short gamma waves. In this chapter, we are interested in the optical wavelengths in the invisible ultraviolet range, 100–300?nm. Each wavelength of the electromagnetic spectrum interacts with the cornea in a specific way. The cornea absorbs ultraviolet and infrared radiation, yet transmits radiation between 300 and 1300?nm. Numerous wavelengths in the range of absorption by the cornea have been tested for PRK, including 193, 248, 3900, and 10,000?nm. This chapter deals mainly with the 193?nm ArF excimer laser, which is the most commonly used laser for corneal refractive surgery.
A collection of atoms, molecules, or ions with unique properties that can emit radiation in the optical part of the electromagnetic spectrum makes up the medium, which can be a gas, liquid, or solid. Commonly used gas media are krypton, carbon dioxide, and ArF, as used in the excimer laser. Most of the chamber is filled with an inert buffer gas such as helium or neon, which fills 88–99% of the cavity to mediate the transfer of energy. The argon constitutes 0.5–12% of the mixture, and the halogen gas such as fluorine constitutes only 0.5–1%.
In any laser, a source of energy must be present that can cause the atoms to undergo a transition from the ground state to the higher energy level ( Fig. 18-1 ). In the excimer laser, electricity is used as the source of energy. The emitted light beam is reflected between two mirrors in the optical resonator. Other atoms are stimulated to a higher energy level by the light, and the light is amplified by reflecting back and forth in the resonator. This creates a much more powerful laser light.
The individual atoms must be stimulated to emit in phase instead of being allowed to emit randomly. This allows the atoms to gain benefits from constructive interference. The radiation that occurs has the same frequency, wavelength, and phase as the stimulating photon or wave.
INTERACTION OF CORNEAL TISSUE AND EXCIMER LASER ENERGY
A unique combination of properties makes excimer lasers appropriate for corneal surgery using 193?nm wavelength light. Excimer lasers operate through a process known as ablative photodecomposition.
Figure 18-1 Schematic configuration of a basic laser. Active laser medium is pumped into the laser cavity from a storage tank. Energy is then discharged into the cavity to stimulate the medium into higher-energy states. As the atoms decay to the lower-energy state, photons are discharged. These then resonate back and forth in the laser cavity and are reflected off the mirrors at the ends of the cavity. This allows amplification of the beam before it is allowed to exit from the partially reflective mirror. The beam is then shaped by various lenses for delivery to the corneal surface.
Most organic materials, including the cornea, have a very strong absorbance for ultraviolet radiation below 300?nm. The higher the absorption of light of a given wavelength, the easier it is for that wavelength to destroy tissue ( Fig. 18-2 ). The ultraviolet photons can directly break chemical bonds because of their high energy. Protein molecules are broken into fragments comprising two or three atoms by ultraviolet radiance. Much of the surplus energy within the system is blown clear of the illuminated area, which minimizes thermal damage to surrounding tissue ( Fig. 18-3 ).
The macromolecules of the cornea, such as the proteins, nucleic acids, and proteoglycans, absorb the most energy when treated with light in the far ultraviolet region, with wavelengths less than 300?nm. The water of the cornea absorbs light energy in both the middle infrared region near 3000–6000?nm and the far infrared region above 10,000?nm ( Figs. 18-4 and 18-5 ).
Figure 18-2 Absorbance versus wavelength for bovine cornea in the far ultraviolet spectrum. The error bar indicates one standard deviation for measurement performed at 193?nm. (From Puliafito CA, Steinert RF, Deutsch TF, et al. Excimer laser ablation of the cornea and lens. Experimental studies. Ophthalmology. 1985;92:741–8.)
Figure 18-3 Debris ejected from the cornea during laser ablation at 193?nm. (From Puliafito CA, Stern D, Krueger RR, Mandel ER. High speed photography of excimer laser ablation of the cornea. Arch Ophthalmol. 1987;105:1255–9.)
The penetration depth of laser light is lower when the absorption of the light is higher. Thermal damage is least when the light penetrates minimally with total absorption. The ArF excimer laser and the fifth harmonic neodymium:yttrium-aluminum-garnet (Nd:YAG) ultraviolet laser have very small penetration depths and can therefore perform corneal surgery with minimal thermal effects. The ArF laser, with its wavelength of 193?nm, creates a more regular margin of excision, with less damage to adjacent tissue, than other wavelengths do.    The quality of ablation with the 193?nm laser is significantly better than that seen with the 248?nm wavelength of the krypton-fluorine laser. 
Ultraviolet light with a wavelength of 193?nm appears to have little if any mutagenic effect on corneal tissue. The reaction of corneal tissue to ultraviolet light, as well as the effect of ultraviolet light on other tissue, may still have some potential for mutagenesis, however.         The risk of mutagenesis from 193?nm light is 1000 to 10,000 times less than the risk from 248?nm radiation, because 248?nm energy is absorbed predominantly by nucleic acids. Concern over mutagenicity has prevented the 248?nm excimer from being used clinically.
LASER RADIAL KERATECTOMY
Laser radial keratectomy involves using the excimer laser to create incisions in the cornea similar to those created with radial keratotomy. This technique is no more accurate than traditional radial keratotomy, because fluid fills the space created during treatment, blocking further pulses. The excisions need to be greater than 30?µm wide to reach an adequate depth. The ablation depth per pulse is dependent on both corneal hydration and the width of the groove, making excimer laser keratectomy less reproducible than radial keratotomy.
EXCIMER LASER KERATOMILEUSIS
Large-area ablation with resculpting of the cornea to correct refractive errors is termed laser keratomileusis. Tissue is removed with great precision, and the corneal epithelium heals over the ablated area to create a smooth surface. Ideally, this occurs without hyperplasia or a significant wound healing response.
Medications such as cocaine, oxybuprocaine, proxymetacaine, and pilocarpine do not appear to change the ablation rates of tissue. About 0.25?µm of tissue is removed with each
Figure 18-4 Absorption spectrum for various molecules in the cornea. The x-axis delineates wavelength. The y-axis indicates the relative absorption. The vertical dotted line represents the 193?nm wavelength corresponding to the ArF excimer laser. At this point, there is a relatively high absorption for both collagen and keratan sulfate. (Adapted with permission from Brightbill FS. Corneal surgery: theory, technique, and tissue, 2nd ed. St. Louis: Mosby; 1993.)
Figure 18-5 Transmission of light through the human cornea. There is minimal transmittance below 300?nm. (From Boettner EA. Invest Ophthalmol Vis Sci. 1962;1:776–83.)
pulse, even in the presence of these agents. Fluorescein decreases the ablation rate by about 40%. Corneal epithelium ablates at a slightly faster rate than corneal stroma. The epithelium is also ablated more irregularly than the stroma, which is the reason why the epithelium is typically removed mechanically before ablation of the stroma. Bowman’s layer ablates about 30% slower than the stroma.
The transition zone between treated and untreated cornea should be as smooth as possible, and the appropriate pattern of tissue removal must occur to obtain the desired refractive effect. The pattern of treatment for the correction of myopia, hyperopia, and astigmatism differs, with myopia requiring greater tissue removal in the center of the cornea, and hyperopia requiring tissue removal from the periphery. Moving slits, constricting and expanding diaphragms, ablatable masks, and computer-controlled application of the laser have been developed to create the appropriate ablation profiles.  Lenses can also be used to shape the beam to the desired configuration ( Fig. 18-6 ).
Figure 18-6 A variety of masking techniques can be used to control the delivery of laser energy to the eye.
EXCIMER LASER SAFETY
Safety equipment for operating room staff includes safety glasses designed to block the wavelength of the 193?nm excimer laser. The fluorine gas used in excimer lasers is extremely toxic in high concentrations, but at the concentrations typically used in laser surgery, it is less so. However, the operating room should have good ventilation to allow the rapid removal of fluorine gas in case of a leak. Undesirable effects of laser exposure are related to total exposure time, duration of the laser pulse, absorption by the body, and wavelength of the energy.
Several 193?nm excimer lasers are commercially available for photorefractive and phototherapeutic keratectomy. Some of these lasers use a large beam diameter of 5–7?mm, whereas others use a scanning technique to deliver a small spot or slit in a computer-controlled manner across the surface of the cornea. All systems include a computer control module with an interactive menu, allowing the development of an ablation protocol for each individual patient or refractive error. Proper positioning of the eye is important, so the excimer laser system should include a microscope with the ability to provide accurate alignment of the eye in all axes. Some lasers have a vacuum apparatus to remove particle debris from the ablation plume. Foot pedal and fingertip controls can be used by the surgeon to manipulate the position of the eye and ablating beam. Even though other wavelengths have been tried, the 193?nm wavelength is totally absorbed by the cornea, causing the breakage of molecular bonds, making it uniquely applicable for corneal treatment (see Figs. 18-2 and 18-4 ).
In the United States, ablation profiles are regulated by the Food and Drug Administration and are restricted to those profiles used in studies leading to the approval of each instrument. As mentioned
Figure 18-7 Illustration of the depth of tissue removal required to correct a myopic refractive error using various ablation zone diameters. (Munnerlyn CR, Koons SJ, Marshall J. Photorefractive keratectomy: a technique for laser refractive surgery. J Cataract Refract Surg. 1988;14:46–52.)
earlier, there are a variety of masking techniques to control the positioning of laser pulses. For instance, the VISX/STAR laser has an expanding diaphragm that the scanning beams are passed through, allowing a larger amount of ablation in the central cornea than in the periphery for the treatment of myopia. For hyperopic ablations, the center is left untreated and the energy is preferentially directed to the midperipheral zones through a scanning system. For astigmatism, expanding slits are used for small zone treatments, and a scanning system is used for larger astigmatic treatments, both of which allow greater treatment along one meridian than another, creating a toric cut.
Other systems use an expanding ring, ablatable mask, rotating mask, or scanning spot of laser energy that preferentially directs energy to the center for myopia and the midperiphery for hyperopia (see Fig. 18-6 ). Some authors now believe that for higher degrees of myopia, multiple treatment zones may be better.   These multizone recipes give a portion of the treatment using large zones of 6–7?mm and various degrees of treatment with several smaller optical zones. This creates a more tapered approach for higher degrees of myopia, reducing the depth of cornea ablated as well as regression and haze. The trade-off in multizone treatment can be visual quality, especially under scotopic conditions. Despite these modifications, high myopia is more difficult to correct than low myopia. 
Myopia Ablation Profiles
As discussed earlier, the amount of tissue that must be removed to produce a certain refractive result is dependent on the optical zone size ( Fig. 18-7 ). The depth of the ablation required to achieve a given refractive result for myopia is approximated by the equation given earlier.
Smaller optical zones are associated with greater degrees of regression, night haloes, and haze. Deeper ablations are associated with greater wound healing and regression, as well as haze. The optical zone size and depth need to be optimized to avoid the excessive wound healing seen in deep ablations and the excessive haloes, edge glare, and irregular astigmatism seen with small optical zones. Larger zones have been shown to be beneficial at reducing scotopic visual complaints.
Astigmatism Ablation Profiles
Astigmatism is present when the refractive properties of the eye are not spherical. This prevents a single focal point from forming. Most astigmatism is regular astigmatism. When regular astigmatism is present, the two principal meridians of refraction are oriented 90° from each other and can be corrected with a spherocylindrical lens. If the two principal meridians are oriented at an angle other than 90° from each other, the astigmatism is termed bioblique or nonorthogonal. When many different meridians of refraction are present, the eye exhibits irregular astigmatism. Bioblique and irregular astigmatism prevent correction with a spherocylindrical lens and require a rigid contact lens for optical correction. Regular astigmatism can be treated surgically by many methods. Irregular astigmatism is still quite difficult to treat surgically, although it is hoped that customized ablations may be helpful in the future.
Surgical correction of astigmatism with the excimer laser requires that the cornea be ablated in a cylindrical or toric pattern. Use of an expandable slit diaphragm, like the one in the original STAR laser, is a common method of astigmatism correction. The newer STAR S3 Variable Spot Scanning System uses a scanning technique to create larger optical zone treatments. In the original system, the excimer laser beams are passed through a set of parallel blades that gradually open as directed by the computer algorithm. The speed of opening depends on the amount of astigmatism to be corrected. The orientation of the blades depends on the orientation of the astigmatism. Deeper ablations occur in the center along the axis of the blades, with gradual transition to shallower ablations at the sides of the slit. Flattening occurs perpendicular to the long axis of the slits. No change in power occurs along the axis of the slits. Thus, for correction of astigmatism in a patient with the refractive error -3.00D +3.00D × 90°, the slits would be oriented horizontally, thus flattening the vertical meridian ( Fig. 18-8 ).
The spherical equivalent is shifted toward the hyperopic side in excimer laser treatment of astigmatism because tissue is removed, causing flattening of the cornea. VISX adds a transition zone to the edge of the long axis to prevent an abrupt change in elevation at the end of the slits.
If the slits and the circular diaphragm open at the same time, an elliptical ablation occurs, correcting compound myopic astigmatism. The rate at which they open can be controlled independently and reflects the amount of myopia and the amount of astigmatism to be corrected. This ablation pattern corrects only regular astigmatism; it is not capable of correcting irregular astigmatism.
Another method of creating a toric ablation utilizes an ablatable mask, such as the system used by Summit. The laser first
Figure 18-8 For correction of with-the-rule astigmatism, the slits are oriented horizontally, allowing flattening of the vertical meridian.
ablates the thinnest areas of the mask, thus allowing greater treatment of the cornea in areas where the mask is thinnest. By differentially protecting the cornea from treatment, any pattern can be created. The mask can be placed directly on the surface of the cornea or along the column delivering the laser beam. The thinnest area of the mask exposes the cornea to the laser beam first; therefore, this region of the cornea will be thinnest at the end of the treatment. Centration is more difficult if the mask is held on the cornea than if it is incorporated into the delivery system.
A computer-controlled scanning beam can also be utilized to treat astigmatism. The Technolas Keracor 116, 117, and 217 lasers, along with the Alcon-Summit LadarVision 4000, utilize a beam that is computer directed to scan along the flat axis to reduce astigmatism. Others such as the Nidek EC-5000 employ a scanning slit in the same fashion. The principle is the same as when an expandable slit is used, in that more laser energy is directed along the flat axis, thereby creating a more spherical cornea. For instance, if the refraction is -3.00D +3.00D × 90°, the beam would travel back and forth along the horizontal axis (see Fig. 18-8 ). The size of the beam can be varied to create a transition zone, preventing a steep step-off at the edges of treatment.
A metal mask placed at the level of the cornea is used in the Aesculap-Meditec MEL 60 excimer laser system. This mask can be rotated to allow treatment of astigmatism with any orientation. The rate at which the mask opens and closes is controlled by the computer, with expanding slits, diaphragms, and spirals allowing concurrent treatment of both myopia and astigmatism or hyperopia and astigmatism.
Hyperopia Ablation Profiles
These ablations use a variety of masking techniques to allow more laser pulses in the midperiphery, with blend zones toward the center and far periphery of the cornea. In some older systems, the beam was passed through multiple diaphragms, allowing a larger amount of ablation in the peripheral cornea than in the center.  Other broad-beam systems use an expanding ring, ablatable mask, or rotating mask. Smaller beam systems, such as scanning slit and spot systems, are directed preferentially to the midperiphery by complex computer programs. The amount of tissue that must be removed to produce a certain refractive result is dependent on the optical zone size.
Patient fixation is critical to the proper placement of laser ablation in all laser systems. As the technology has expanded, allowing the treatment of hyperopic astigmatism and the design of custom corneal treatments for higher-order ablations, the accuracy of laser placement has become more critical. This is especially true in lasers utilizing small scanning spot systems. All patients show evidence of saccadic movement, including small refixation movements, during periods of fixation. Patients also vary in their ability to cooperate with fixation.
Several laser manufacturers offer laser tracking systems to increase the accuracy of shot placement. Tracking systems may improve the quality of ablations and the accuracy of centration and provide greater smoothness.  Tracking systems vary in sampling frequency from 60?Hz for camera-based systems such as in the VISX Active-Trak system to 4000?Hz for the laser-radar system in the LadarVision 4000. The active tracking systems can be very useful in treating patients with head tremors, tics, and nystagmus. Tracking is an excellent adjunct to laser vision correction, but it is no substitute for proper and attentive patient fixation. Tracking systems will undoubtedly assume greater importance as attempts are made at mapping the treatment of higher-order aberrations.
Examination and Counseling
The risks and benefits for each individual patient should be assessed, as with all refractive surgical procedures. Visual acuity should be measured with a careful manifest and cycloplegic refraction, ocular dominance testing, and distance and near vision with and without correction. Anterior segment and posterior segment examinations should be performed to rule out other conditions that might adversely affect the surgical result. Pachymetry measurements are performed to make certain that the cornea is of normal thickness. Computerized topographical analysis can help screen for subclinical keratoconus or other corneal diseases.
Relative contraindications to laser treatment include advanced diabetes, collagen vascular disease, previous herpes (simplex or zoster) infection, severe dry eye, untreated blepharitis, neurotrophic cornea, peripheral ulcerative keratitis of any cause, and patients on the following medications: isotretinoin (Accutane), amiodarone (Cordarone), or sumatriptan (Imitrex).
The most important aspect of treating presbyopic patients is preoperative counseling. The patient’s goals for increased spectacle independence must be fully understood. For example, a 20-year-old patient who has a result of plano in each eye with 20/20 uncorrected distance acuity will function well for all tasks, including near vision. A 50-year-old patient with the same results can have good distance function but will become significantly more dependent on correction for near vision. A careful understanding of the patient’s goals and vocational demands will help prevent disappointment postoperatively. Target refraction should be carefully discussed with every patient, but this is critical in presbyopes.
Monovision should be carefully discussed with all presbyopes. Monovision success rates vary from 33–50%, so a trial in contact lenses is usually appropriate before laser vision correction. Generally, a 39-year-old pre-presbyope will not tolerate 3D of anisometropia to allow monovision. Even a 49-year-old presbyope will have difficulty accepting this condition. In general, a 40-year-old will tolerate -0.75 to -1.00D of anisometropia, a 50-year-old will tolerate monovision of -1.25 to -1.50D, and a 60-year-old will tolerate -1.50 to -1.75D of residual myopia in the nondominant eye. When attempting levels of anisometropia greater than this, it is mandatory to try the correction in contact lenses first to make certain that the patient is truly tolerant of this degree of anisometropia. Some patients may be at a level of myopia such that treating only the dominant eye and leaving the nondominant eye myopic would be appropriate.
Figure 18-9 This patient has a poor blink reflex and, after the administration of anesthetic drops, has thinning of the inferior cornea. This can lead to greater tissue removal inferiorly and an asymmetrical ablation. Care must be taken to ensure even hydration.
There are many variations in the technique for excimer PRK. This section describes a typical technique, but it is important for each surgeon to keep his or her technique as consistent as possible and to monitor the results. Attention to consistency in technique as well as to ambient conditions such as temperature and humidity can have a profound effect on refractive results. The development of a personalized nomogram to allow for variations in individual technique, laser variations, and differences in altitude or climate is recommended. The cornea’s state of hydration when treatment occurs plays a large role in the refractive result after excimer laser vision correction, including PRK and laser in situ keratomileusis (LASIK).
For PRK, the spectacle correction is adjusted to the corneal plane to take into account vertex distance. On some older systems, this must be done by looking at a chart or table; in newer systems, the computer software performs the calculation. If astigmatism is to be treated, the minus cylinder format is helpful, but in many systems, positive cylinder can be used if this is more convenient.
Preoperatively, the patient should receive anesthetic drops. It is important to perform the treatment as soon as possible after the drops are instilled to prevent exposure keratitis from poor blinking, which will dry and thin the cornea. This can lead to overcorrection or an asymmetrical ablation. Drying of the inferior half of the cornea, as often occurs after anesthetic drops have been instilled, can lead to increased thinning inferiorly ( Figs. 18-9 and 18-10 ).
The patient is positioned under the microscope, carefully aligning the head to make sure that the iris plane is perpendicular to the laser beam. The eyelids are prepared with dilute povidone-iodine (Betadine) solution. The conjunctival fornices can be prepared with Betadine drop preparations or broad-spectrum antibiotic drops. A lid speculum is inserted to open the eyelids. Careful centration with the eye aligned in the x, y, and z planes is crucial. Centration on the pupil is preferred by some but continues to be an area of controversy. 
The epithelium is marked with a 7 or 8?mm optical zone marker for myopia or a 9 or 10?mm marker for hyperopia, again centering on the pupil. The epithelium is bluntly removed with a Tooke knife (Storz Ophthalmics, St. Louis, MO), a #64 or #69 Bard-Parker blade (Bard-Parker, Franklin Lakes, NJ), Visitec disposable excimer spatula (Visitec Company, Sarasota, FL), or rotating brush (Amoils Epithelial Scrubber; Innova, Inc., Toronto, Canada). Alcohol can also be used to loosen the epithelium. It is typically used as a 20% concentration of ethanol and is applied
Figure 18-10 Elevation topography of a patient with a poor blink reflex after the administration of anesthetic drops. Relative depression of the cornea can be seen inferiorly with irregular astigmatism.
for 20–30 seconds. The area of epithelial removal depends on the total ablation diameter, which is greater for the treatment of hyperopia. It is important to remove the epithelium totally, so that only Bowman’s membrane remains. Any residual epithelium will create an uneven ablation and irregular astigmatism.
It is also possible to remove the epithelium with the excimer laser. The laser should be set to a depth of approximately 50?µm, with the beam set to its widest aperture. The excimer laser is centered, and the ablation is begun. The microscope light is dimmed so that an area of fluorescence is seen where the epithelium is being ablated by the laser beam. The ablation is stopped when a change from a fluorescent pattern to a dark pattern is seen, indicating that the epithelium has been ablated. The epithelium may be more or less than 50?µm deep. If there is still fluorescence across the whole area after a 50?µm ablation has been performed, an additional depth of 25?µm should be set for the laser, stopping the treatment when all the epithelium has been removed. It is often difficult to tell exactly when all the epithelium has been removed by visual clues alone.